1. Field of the Invention
The present invention relates to an optical scanning device which performs high speed deflection scanning of a light beam such as a laser light or the like in an image forming apparatus such as a laser printer or the like.
2. Description of the Related Art
The principle of an optical scanning device of this kind will be explained with reference to FIG. 9. A laser beam (light beam) L emitted from a laser beam generator (light source) 1 passes through a cylindrical lens 2 to be collimated into a predetermined beam shape, and the collimated laser beam L is incident on reflecting facets of a rotary polygonal mirror 3 with a regular polygonal shape rotating in the direction of an arrow to be deflection-scanned repeatedly, so that the scanned light is focused on a member 5 to be scanned through imaging lens systems 4a and 4b. 
In such an optical scanning device, however, it is known that a whirring sound from the rotary polygonal mirror is transmitted to the outside as noise. It is believed that the noise occurs because the rotary polygonal mirror has a regular polygonal shape, an air flow towards an outer peripheral direction occurs around the rotary polygonal mirror due to a difference in peripheral velocity during rotation of the polygonal mirror between corner sections and flat reflecting facets thereof (the corner portion has a peripheral velocity larger than that of the reflecting facet), and further, because the air flow develops into a turbulent flow. In recent years, since a rotary polygonal mirror which rotates at a high speed such as ten and several thousand rpm to twenty thousand rpm or more according to a high speed orientation of a processing speed is provided, the noise tends to increase. Therefore, various measures for suppressing noise generation have been taken.
As these measures, there is a structure in which noise is isolated by covering a rotary polygonal mirror with a cover (for example, disclosed in JP-A Publication No. 06-331919), a structure where noise is reduced by a cylindrical member provided around a rotary polygonal mirror (for example, disclosed in JP-A Publication No. 10-148784), and the like. Also, as a structure in which a member is provided around a rotary polygonal mirror, the present applicant has proposed an optical scanning device where a cylindrical member having a corrugated inner surface undulated in a rotating direction is arranged around the rotary polygonal mirror, as known from JP-A Publication No. 2000-284208.
A conventional art which isolates noise by covering a rotary polygonal mirror with a cover is not constructed so as to cancel an turbulent flow of air which is a noise source, but a problem occurs in the art in which the turbulent of air is generated and vibrates the cover. Also, like the above, even when a cylindrical member is arranged around a rotary polygonal mirror, there is a possibility that noise and/or vibrations due to a new air flow may be generated by the cylindrical member. On the other hand, the above conventional art of the present applicant is directed to suppressing rotation unevenness of the rotary polygonal mirror, but it cannot be expected that a noise suppressing effect will be achieved by the conventional art.
Accordingly, an object of the present invention is to provide an optical scanning device which can suppress noise generated from a rotary polygonal mirror effectively, thereby allowing a high speed orientation sufficiently.
The present invention, which has been made for achieving the above object, comprises a rotary polygonal mirror which performs deflection scanning of a light beam emitted from a light source, and an annular member which is arranged around the rotary polygonal mirror in proximity thereto and which has a width extending generally in a diametrical direction of the rotary polygonal mirror.
According to the present invention, an air flow generated around the rotary polygonal mirror according to rotation of the rotary polygonal mirror strikes on the annular member to be split and rectified. That is, since the air flow is split and rectified before it develops into a turbulent flow, occurrence of a turbulent flow becomes difficult so that noise occurrence is suppressed.
In order to split an air flow securely, the annular member according to the present invention is preferably formed in a shape having a fixed width extending in a diametrical direction of the rotary polygonal mirror, and, for example, a flat ring can be employed as the annular member. Therefore, an edge on a side of an inner diameter of such an annular ring, i.e., an inner peripheral edge of ring opposed to the rotary polygonal mirror, is formed concentrically with the rotary polygonal mirror, and the relationship between a radius r of the inner peripheral edge and a diameter d of the circumscribed circle of the rotary polygonal mirror preferably meets r/d=0.6 to 1, more preferably, r/d=0.6 to 0.7. It has been determined experimentally that the turbulent flow occurs in a range of r/d=0.6 to 1 around the rotary polygonal mirror, and it is therefore desirable to arrange the inner peripheral edge of the annular ring in the range for suppressing occurrence of a turbulent flow.
Also, in order to secure a function of splitting an air flow, it is preferable that the annular member be disposed almost at a center in a direction of the height of the reflecting facet of the rotary polygonal mirror. When the annular member is arranged so as to be positioned eccentrically to one side in the direction of the height of the reflecting facet of the rotary polygonal mirror, there is a possibility that an air flow in a space positioned on the other side where the annular member is not disposed may develop into a turbulent flow, and it is therefore difficult to achieve an effective splitting function of the air flow. Furthermore, such an aspect in which the sectional configuration of the annular member is formed in an almost wedge shape where the annular member has an acute angle on the inner peripheral edge can preferably be employed because the air splitting function can be ensured.
Furthermore, the number of the annular members may be one or may be more than one. In the case of using plural annular members, when such a structure is adopted in which these annular members are disposed at intervals in the direction of the height of the reflecting facet of the rotary polygonal mirror in a stacked manner, an air flow is split and rectified finer so that improvement in noise suppressing effect can be achieved. In the case in which the plural annular members are arranged in a stacked manner, when the annular members are arranged in the stacked manner such that the radiuses r of the inner peripheral edges of adjacent annular members are different from each other within the range of r/d=0.6 to 1 in the relationship between the radius r and the diameter d of the circumscribed circle of the rotary polygonal mirror, namely, the inner diameters of adjacent annular members are different from each other, an air flow can be received on faces of the annular members exposed to the rotary polygonal mirror so that the rectifying effect can further be improved.
Also, in the aspect where the plural annular members are arranged in the stacked manner, the sectional configuration of at least one of the annular members may be formed in an almost wedge shape. In this case, since an air flow path between an annular member with a wedge-shaped sectional configuration and an annular member adjacent thereto (this member may also have a wedge-shaped sectional configuration) is gradually narrowed, and an air flow is made thinner in a pressing manner so that its development into a turbulent flow is suppressed.
Furthermore, the annular member of the present invention includes one that it is formed in a spiral shape. In this case, an air flow occurring around the rotary polygonal mirror is split by the plural annular members arranged along the direction of the height of the reflecting facet of the rotary polygonal mirror to be passed through a spiral groove between the annular members so that a rectifying effect can be achieved.